Birefringence compensated integrated optical switching or modulation device

Abstract

A planar waveguide Mach-Zehnder interferometer (MZ) has improved performance as a variable optical attenuator as well as a thermo-optic switch (TOS) with reduced polarization dependent loss (PDL) in high attenuation states. The PDL was reduced by correcting for the increased birefringence that occurs on heating one waveguide arm by making the two waveguides inherently asymmetric, such that when one of the waveguides is heated, the differential birefringence of the two arms becomes zero. This asymmetry can be realized simply by changing the width of one of the waveguides. The modified device allows for very small PDL in the high attenuation state of a VOA and for theoretically perfect contrast in a TOS in the off state, with only a small penalty in PDL of the on state.

What is claimed is:  
     
       1. An integrated optical Mach-Zehnder interferometer device comprising: 
       a first and second channel waveguide disposed on a substrate, each channel waveguide having;  
       an input branch for receiving an incident light beam arriving from an input port,  
       an output branch for directing an incident light beam to an output port,  
       an arm segment connecting the input branch and the output branch,  
       an input coupler connecting the input branch of at least one channel waveguide to the arm segments of the first and the second channel waveguides,  
       an output coupler connecting the output branch of at least one channel waveguide to the arm segments of the first and the second channel waveguides, whereby the input coupler splits the light beam between the first and second arm segments and the output coupler recombines the split light beams arriving from the first and second arm segments,  
       at least one actuator for modulating the refractive index of at least one of the first and second channel waveguide arm segments  
       at least one of the first and second arm segments having an initial birefringence Δ of equal magnitude to the birefringence induced in the other of the first and second arm segments by operation of the actuator.  
     
     
       2. An integrated optical Mach-Zehnder interferometer device according to  claim 1  wherein said at least one actuator is a thin film heater that modulates the refractive index by increasing the temperature of either the first or second arm segment. 
     
     
       3. An integrated optical Mach-Zehnder interferometer device according to  claim 1  wherein the first and second arm segments have an unequal physical path length such that the optical path length is substantially equal when the device is not actuated. 
     
     
       4. An integrated optical Mach-Zehnder interferometer device according to  claim 3  wherein said at least one actuator is associated with the first channel waveguide and the second channel waveguide has an initial birefringence Δ. 
     
     
       5. An integrated optical Mach-Zehnder interferometer device according to  claim 3  wherein said at least one actuator is associated with the first channel waveguide and the first channel waveguide has an initial birefringence Δ. 
     
     
       6. An integrated optical Mach-Zehnder interferometer device according to  claim 4  wherein the initial birefringence Δ is achieved by providing a first or second channel waveguide having a channel width greater than the channel height. 
     
     
       7. An integrated optical Mach-Zehnder interferometer device according to  claim 5  wherein either the first or second channel waveguide has a width of less than six microns and the ratio between width and height of the first or second channel waveguide is greater than 1.1:1. 
     
     
       8. An integrated optical Mach-Zehnder interferometer device according to  claim 4  wherein the width of the first channel waveguide is less than the width of the second channel waveguide. 
     
     
       9. An integrated optical Mach-Zehnder interferometer device according to  claim 1  wherein the first and second channel waveguides have a different width to provide an initial birefringence Δ. 
     
     
       10. An integrated optical Mach-Zehnder interferometer device comprising: 
       a) a first and second channel waveguide disposed on a substrate, each channel waveguide having;  
       an input branch for receiving an incident light beam arriving from an input port  
       an output branch for directing an incident light beam to an output port,  
       an arm segment connecting the input branch and the output branch,  
       b) an input coupler connecting the input branch of at least one channel waveguide to the arm segments of the first and the second channel waveguides,  
       c) an output coupler connecting the output branch of at least one channel waveguide to the arm segments of the first and the second channel waveguides, whereby the input coupler splits the light beam between the first and second arm segments and the output coupler recombines the split light beams arriving from the first and second arm segments,  
       d) a thin film heater for modulating the temperature of either the first or second arm segment,  
       e) at least one of the first and second arm segments having an initial birefringence Δ of equal magnitude to the birefringence that develops on heating the first or second arm segment,  
       wherein energizing the thin film heater results in an input signal attenuation of greater than 20 decibels with a polarization dependent loss of less than 1.5 decibels. 
     
     
       11. An integrated optical Mach-Zehnder interferometer device according to  claim 10  wherein energizing the thin film heater results in an input signal attenuation of greater than 25 decibels with a polarization dependent loss of less than 1.5 decibels. 
     
     
       12. An integrated optical Mach-Zehnder interferometer device according to  claim 10  wherein energizing the thin film heater results in an input signal attenuation of greater than 20 decibels with a polarization dependent loss of less than 1.0 decibels. 
     
     
       13. An integrated optical Mach-Zehnder interferometer device according to  claim 10  wherein energizing the thin film heater results in an input signal attenuation of greater than 20 decibels with a polarization dependent loss of less than 0.5 decibels. 
     
     
       14. An integrated optical Mach-Zehnder interferometer device according to  claim 10  wherein the first and second arm segments have an unequal physical path length such that the optical path length is substantially equal when the thin film heater is not energized. 
     
     
       15. An integrated optical Mach-Zehnder interferometer device according to  claim 3  wherein the channel waveguides are formed of a glass comprising silica on a planar substrate consisting essentially of silicon. 
     
     
       16. An integrated optical Mach-Zehnder interferometer device according to  claim 3  wherein the device is a variable optical attenuator. 
     
     
       17. An integrated optical Mach-Zehnder interferometer device according to  claim 3  wherein the device is an optical switch. 
     
     
       18. A process for modulating the intensity of an optical signal propagating within an integrated optical Mach-Zehnder interferometer having at least first and second waveguide arm segments, the process comprising: 
       a) providing a variable stimulus to a first actuator associated with a first waveguide arm segment to increase the effective refractive index in the first arm segment,  
       b) providing a variable stimulus to a second actuator associated with a second waveguide arm segment to modulate the birefringence of the second waveguide arm segment of similar magnitude and sign to the birefringence induced in the first arm segment by the first actuator.  
     
     
       19. An integrated optical Mach-Zehnder interferometer device according to  claim 1 , the device further comprising: 
       a third and fourth channel waveguide disposed on the substrate, each channel waveguide having;  
       an input branch for receiving an incident light beam arriving the first or second channel waveguides;  
       an output branch for directing an incident light beam to an output port;  
       an arm segment connecting the input branch and the output branch;  
       a second input coupler connecting the input branch of at least one of said third and forth channel waveguides to the arm segments of said third and fourth channel waveguides,  
       a second output coupler connecting the output branch of at least one of said third and fourth channel waveguides to the arm segments of the third and the fourth channel waveguides, whereby the second input coupler splits the light beam between the third and fourth arm segments and the second output coupler recombines the split light beams arriving from the third and fourth arm segments,  
       an actuator for modulating the refractive index of at least one of the first, second third and fourth channel waveguide arm segments  
       at least one of the third and fourth arm segments has a birefringence Δ of equal magnitude to the birefringence induced in the first or second arm segment by operation of said actuator.  
     
     
       20. An integrated optical Mach-Zehnder interferometer device according to  claim 1 , wherein the actuator is a thin film heater that modulates the refractive index by increasing the temperature of either the first or second arm segment wherein the device is a variable optical attenuator. 
     
     
       21. An integrated optical Mach-Zehnder interferometer device according to  claim 1 , wherein the actuator is a thin film heater that modulates the refractive index by increasing the temperature of either the first or second arm segment wherein the device is a thermo-optical switch. 
     
     
       22. An integrated optical Mach-Zehnder interferometer device according to  claim 19  wherein said actuator modulates the refractive index of at least one of the third and fourth waveguide arm segments. 
     
     
       23. An integrated optical Mach-Zehnder interferometer device according to  claim 19  wherein said actuator modulates the refractive index of at least one of the first and second waveguide arm segments and at least one of the third and fourth waveguide arm segment has an initial birefringence Δ. 
     
     
       24. An integrated optical Mach-Zehnder interferometer device according to  claim 19  further comprising an actuator that modulates the birefringence of at least one of the second, third and fourth arm segments.